1. Field of the Invention:
The present invention relates to a turbine, and in particular relates to a steam turbine and a gas turbine wherein the turbine blades are mounted on a rotor disk.
2. Discussion of the Background:
FIG. 9 is a diagrammatic axial cross-sectional view showing a conventional turbine. In the turbine shown in FIG. 9, a large number of stages 4 consisting of nozzles 2 and turbine blades 3 (hereinbelow simply called "blades") are provided along the axial direction of a rotor disk 1. Turbine nozzles 2 are formed by nozzle plates 5 constituting fluid passages, a nozzle outer ring 6 onto which these nozzle plates 5 are fixed from the outer side, and a nozzle diaphragm inner ring 7 onto which nozzle plates 5 are fixed from the inner side. The nozzles 2 are supported on a casing 8 through the nozzle outer ring 6 that is fitted into a circular groove 9 provided on the inner circumference of the casing 8. As shown in FIG. 10, the blade 3 consists of an effective blade portion 10 through which the operating fluid flows, a dovetail-shaped anchoring portion 11 provided at the bottom of the effective blade portion 10, and a tenon 12 provided at the top of the effective blade portion 10. These blades 3 are mounted on the rotor disk 1 by fitting anchoring portions 11 from the circumferential direction of the rotor disk 1 into grooves 13 formed through the outer circumference of the rotor disk 1. The blades 3 are mounted with a prescribed separation in the peripheral direction around the entire circumference of the rotary disk 1 and are linked together by shrouds 14, which are mounted as shown in FIG. 9 at the outer circumference of the blades 3 by caulking tenons 12. The flow direction of the operating fluid is shown by arrow A in FIG. 9.
However, in the conventional turbine constructed in this manner, during operation of the turbine, wakeflow, including slow flow containing eddies coming from the trailing edge of nozzle plates 5 of the upstream nozzle 2, reaches the effective portions 10 of the blades. The velocity distribution of the wakeflow of these nozzle plates 5 is diagrammatically shown in FIG. 11. The non-uniform flow, including a lower-velocity portion, represented by the wakeflow causes effective portions 10 of the blades to receive an excitation pulsating force each time they pass through the pitch interval of the nozzle plates 5. This excitation frequency is expressed by the relationship equal to: EQU (number of nozzle plates).times.(velocity of rotor rotation).
Blades 3 resonate if this excitation frequency equals any of the resonant frequencies of blades 3. If such resonance occurs, this subjects the blades 3 to high vibrational stresses, risking local damage or failure.
Conventionally, to avoid resonance, the number of nozzle plates 5 was selected such that none of the resonant frequencies of the blades 3 would coincide with the excitation frequency. However, it is difficult to accurately predict the resonant frequencies of the blades 3. Another problem was that turbine efficiency was adversely affected by the need to select the number of nozzle plates 5 such that none of the resonant frequencies of the blades 3 would coincide with the excitation frequency. Due to this restriction imposed on the number of nozzle plates 5 which can be utilized, the nozzle plates 5 were not disposed circumferentially at the optimum pitch to give the highest efficiency.